The present invention relates to an automotive audio reproducing apparatus.
When an audio reproducing apparatus reproduces music or the like, ideal height of a reproduced sound image is said to be eye level of the listener. Therefore, speakers are generally mounted at the eye level of the listener.
However, with an automotive audio reproducing apparatus, it is difficult to mount speakers at the eye level of the listener (driver or passenger of the vehicle, that is, an occupant). As shown in FIG. 12A, the speakers are often mounted at a lower position (1) of front doors of the vehicle or at a lower position (2) of rear doors. Hence, reproduced sound is heard from the direction of the lower position, so that the sound image is localized at a position lower than the eye level of the listener.
In order to avoid such a problem, there is a method of mounting speakers of small diameter for reproducing high frequencies at a position (3) in front of the listener, as shown in FIG. 12A. However, this method causes reproduced sound at high frequencies and reproduced sound at low frequencies to be outputted from different positions, thereby resulting in separate reproduced sounds.
Also, it is known that sound tends to be absorbed more as its frequency is increased. Therefore, when the speakers are mounted at a lower position in a passenger compartment, high-frequency sound is absorbed by the seats and the interior of the compartment. This results in a difference between the reproduced sound outputted by the audio reproducing apparatus and the sound actually heard by the listener.
In order to deal with the above situation, it is effective to actually determine transfer functions in the compartment and correct the reproduced sound according to the transfer functions. However, this requires a high-performance digital signal processing apparatus. Since such a digital signal processing apparatus is rather expensive, it is difficult to use it in a consumer audio reproducing apparatus.
In addition, when the reproduced sound is corrected according to transfer functions, high frequencies generally tend to be emphasized. Therefore, when sound volume level is increased, high-frequency sound becomes excessively noticeable.
Furthermore, the passenger compartment is rather small when viewed as an acoustic space, and therefore affects the reproduced sound. Thus, the reproduced sound actually heard by the listener lacks in breadth and depth.
The present invention has been made to solve the above problems.
Thus, according to the present invention, there is provided an automotive audio reproducing apparatus comprising:
a sound image position correction circuit for converting a left-channel input digital audio signal XL(Z) and a right-channel input digital audio signal XR(Z) into a digital audio signal YL(Z) and a digital audio signal YR(Z), respectively, for output expressed by:
YL(Z)xc2x7GLL(Z)+YR(Z)xc2x7GLR(Z)=XL(Z)xc2x7FLL(Z)+XR(Z)xc2x7FLR(Z)
YR(Z)xc2x7GLL(Z)+YL(Z)xc2x7GLR(Z)=XR(Z)xc2x7FLL(Z)+XL(Z)xc2x7FLR(Z)
xe2x80x83where
FLL(Z) is a head related transfer function from a first left-channel speaker and a first right-channel speaker located in front of a listener in a passenger compartment to a left ear and a right ear of the listener, respectively;
FLR(Z) is a head related transfer function from the first left-channel speaker and the first right-channel speaker to the right ear and the left ear of the listener, respectively;
GLL(Z) is a head related transfer function from a second left-channel speaker and a second right-channel speaker located in lower front of the listener to the left ear and the right ear of the listener, respectively; and
GLR(Z) is a head related transfer function from the second left-channel speaker and the second right-channel speaker to the right ear and the left ear of the listener, respectively;
a reflected sound signal generating circuit for generating reflected sound signals by delaying the output signals YL(Z) and YR(Z), respectively;
a pair of adding circuits for adding the reflected sound signals to the output signals YL(Z) and YR(Z), respectively; and
a D/A converter circuit for being supplied with output signals of the pair of adding circuits;
wherein when
Hp(Z)=(FLL(Z)+FLR(Z))/(GLL(Z)+GLR(Z))
Hm(Z)=(FLL(Z)xe2x88x92FLR(Z))/(GLL(Z)xe2x88x92GLR(Z)),
the sound image position correction circuit includes:
a first adding circuit and a first subtracting circuit for subjecting the input digital audio signals XL(Z) and XR(Z) to addition and subtraction, respectively;
a first digital filter and a second digital filter having transfer characteristics of the Hp(Z) and the Hm(Z) for being supplied with output signals of the first adding circuit and the first subtracting circuit, respectively;
a second adding circuit and a second subtracting circuit for subjecting output signals of the first digital filter and the second digital filter to addition and subtraction and thereby generating the output signals YL(Z) and YR(Z), respectively; and
a level control circuit connected in series with the second digital filter in a signal line between the first subtracting circuit and the second adding circuit and the second subtracting circuit;
whereby the level control circuit controls level of a difference signal supplied to the second adding circuit and the second subtracting circuit; and
analog signals outputted from the D/A converter circuit are supplied to the second left-channel speaker and the second right-channel speaker, respectively.
Thus, virtual speakers are disposed in front of the listener, and the virtual speakers reproduce a sound field and a sound image.